I don't think Cain has anything to do with Leah being possessed. People have become corrupt before without much interference, like Leoric, Leah's grandfather. I think she either puts the soulstone in herself, or someone else does.
Seems like a nice spot for the black soulstone to fit on her chest. Not sure she stays possessed though, maybe someone steals the stone afterwards and she's just undead or something.
Just looked at the Top 10 most anticipated game Video on the Frontpage... and i noticed something quite revealing on the video when we see Leah at the end just after we see diablo Screaming.
Looking back at the video i noticed that Leah isn't holding her chest but the amulet. I had the theory that it's a gift from Aiden to Adria. But even if that's not true it MUST have something to do with Diablo. It's on the side and wax-sigil of the book of Cain and also seen in the Black Soulstone Cinematic. Guess someone has to search the trinket data for it and we will know more...
I'm just curious by the way the cinematic is cut. Diablo is shown directly before Leah grabs the Amulet. If the essence of a prime evil can be trapped in a "worldstone-shard" why not in an amulet? Especially when it comes from either Adria or Aiden... This is prolly the wrong thread for aussumptions but It could also be a charm of some sort to protect her, or a key to finding the black soulstone. Damn, I want ANSWERS!
Well apparently we crushed diablo's body (aiden, and dropped burried him in lava to honor him?) and went back and crushed his soulstone on the hellforge after we vanquished him in d2... Which if I'm correct just leaves him without a body, meaning his soul, being a piece of other living beings (VOLDEMORRTTTTT) returns to the void until it can(chooses to..?) make a connection with something, somewhere. Adria or leah most likely have something to do with diablo's return, intentionally or not. With that said I really doubt/hope diablo doesn't take over leah, though it can't be discounted as a possibility. I like my diablo's meaty and bulky!
If Diablo absorbed the power from all the Great Evils, his feminine legs could come partially from Andariel and not Leah. I still don't think he will posses Leah because he really doesn't have to since there is no more Worldstone, and even the devs kept telling in the panels and interviews that we haven't seen the true form of Diablo - yet. However, something fishy is going to happen with Leah, there's no doubt about that.
And speaking of Andariel and Duriel, when someone asked about them during the Q&A, Metzen said: I wouldn't leave them lying around. They're dangerous! So why not absorb their asses to gain more power.
That conception of the story is so good, I don't understand why people didn't talk about it after your post. And the fact that Diablo use curses from Ball/Mephisto/Duriel/Andariel make it much more believable. And it would explain diablo's new form and the three heads part.